The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present invention is related in general to equipment for servicing subterranean wells. Particularly, the invention relates to an apparatus and method for remotely launching cementing plugs during the primary cementation of a subterranean well.
Most primary cementing treatments involve the use of wiper plugs that travel through the interior of a tubular body (e.g., casing or liner). When launched, the plugs travel from the top of the tubular body to the bottom, where they become seated. The purpose of the plugs is to separate and prevent commingling of different fluids during their journey through the tubular body. In most cases, operators deploy a bottom plug and a top plug.
After the tubular body is installed in the wellbore, the annulus between the tubular body and the wellbore wall (or another tubular body) is usually filled with drilling fluid. When the primary cementing treatment commences, the bottom plug is first launched into the tubular body, followed by the cement slurry. The cement slurry may be preceded by a spacer fluid, a chemical wash or both. The function of the bottom plug is to scrape traces of drilling fluid from the internal surface of the tubular body, and to prevent contact between the drilling fluid and the cement slurry.
The bottom-plug launching and conveyance through the tubular body arises from pressure applied by the cement slurry. When the bottom plug completes its journey through the tubular body, it becomes seated on float equipment installed at the bottom of the tubular body. Continued pumping exerts sufficient pressure to rupture a membrane at the top of the bottom plug, allowing the cement slurry to flow through an interior passage in the bottom plug, exit the bottom of the tubular body and continue into the annulus.
After sufficient cement slurry to fill the annulus has been pumped into the tubular body, the top plug is launched into the tubular body, and a displacement fluid is pumped behind the plug. The displacement fluid forces the plug through the tubular body. Displacement fluids may comprise (but not be limited to) water, spacer fluids and completion fluids. The function of the top plug is to scrape traces of cement slurry from the internal surface of the tubular body, isolate the cement slurry from the displacement fluid and, upon landing on the bottom plug, seal the tubular body interior from the annulus. Unlike the bottom plug, the top plug has no membrane or interior passage through which fluids may flow.
A thorough description of the primary cementing process and the equipment employed to perform the service may be found in the following references. (1) Piot B. and Cuvillier G.: “Primary Cementing,” in Nelson E. B. and Guillot D. (eds.): Well Cementing-2nd Edition, Houston: Schlumberger (2006): 459-501. (2) Leugemors E., Metson J., Pessin J.-L., Colvard R. L., Krauss C. D. and Plante M.: “Cementing Equipment and Casing Hardware,” in Nelson E. B. and Guillot D. (eds.): Well Cementing-2nd Edition, Houston: Schlumberger (2006): 343-434.
Wiper plugs are usually launched from a cementing head that is attached to the tubular body near the drilling rig. The tubular body rises from the bottom of the openhole to the rig floor. However, for subsea completions, the problem becomes more complicated, and fluid isolation becomes more and more critical as water depth increases. It thus becomes impractical to launch wiper plugs from the surface. Therefore, the cementing head containing the wiper plugs rests on the seafloor, and the top of the tubular body ends at the mudline. Drillpipe connects the top of the tubular body to the rig floor on the surface. During the cementing process, darts are released into the drillpipe on surface, travel through the drillpipe to the seafloor and, upon arrival, trigger the release of the wiper plugs.
After the first dart is launched, cement slurry is pumped behind it. When the first dart lands inside the cementing head, the bottom plug is released. The second dart is launched after sufficient cement slurry has been pumped to fill the annulus. A displacement fluid is pumped behind the second dart. When the second dart arrives, the top plug is released. A brief peak in surface pressure indicates when each wiper plug has been launched. This process is detailed in the following references: (1) Buisine P. and Lavaure G.: “Equipment for Remote Launching of Cementing Plugs into Subsea Drilled Wells,” European Patent Application 0 450 676 A1 (1991); (2) Brandt W. et al.: “Deepening the Search for Offshore Hydrocarbons.” Oilfield Review (Spring 1998) 10, No. 1, 2-21.
Those skilled in the art will understand that process fluids may comprise drilling fluids, cement slurries, chemical washes, spacer fluids and completion fluids.
Previous plug-launching systems are configured such that the length of the dart must match the length of the plug being launched. The arrival and displacement of the dart inside the cementing head causes a rod and piston to likewise move downward into the plug basket. The distance the rod and piston move downward is equal to the axial displacement distance of the dart. The cementing-plug length may vary depending upon the casing size into which it is being launched. Therefore, it is necessary for the operator to have various sizes of darts available.
The necessity for the dart length to be equal to the plug length may also pose ergonomic problems. When longer plugs are employed, the length of the dart launching apparatus may be difficult to handle on offshore facilities.
Despite the valuable contributions of the prior art, it remains desirable, therefore, to provide an improved apparatus and methods for launching various sizes of cementing plugs.